The present invention relates to an electronic device for remotely displaying information on a plurality of displays, in particular a device employing the infrared (IR) transmission between the central control unit and the remote displaying units whereas the radio frequency (RF) transmission is employed in the opposite direction.
A typical application of said kind of device is in supermarkets and the like, where the variation of the prices of the exhibited products involves a long manual work for replacing the corresponding labels positioned on the shelves. Various remote displaying systems have been proposed to avoid this waste of labour, essentially consisting of a plurality of displays (typically LCD) located on the shelves as a replacement for the conventional labels and updated by means of IR, RF, ultrasonic, etc. signals emitted by a plurality of transmitters connected to a central control unit. In this way, it is sufficient to introduce the new list of prices in the central unit so that it can transmit them to the corresponding electronic "labels" in an automatic way, without any need of further human intervention. For the sake of simplicity, reference will be made hereafter to this kind of application, though it is clear that what will be said can be transferred to other similar applications.
The transmitters are usually connected by wire to the central unit, preferably using the electric cables already present for the lighting, and they make up a network of communication "nodes" each of which controls a group of labels. Furthermore, each label has an own specific "address" within a single group, so that it is possible to act separately on each single label. To this purpose, the data transmission begins with a code which identifies the label or the group of labels to which the message is directed.
For a greater system reliability it is absolutely preferable that the success of the data transmission to the labels is acknowledged by means of a reply message emitted by the latter towards their node. In order to do so, each label as well as each node must be provided with a transceiving apparatus. This allows to overcome the limits of one-way systems, such as for example those disclosed in U.S. Pat. No. 4,766,295 and in the international (PCT) patent applications WO 88/06773 and WO 90/13067. In particular, while in the first two above-cited cases the transmission takes place in IR or RF, therefore with the theoretical possibility of providing the labels with a reply transmitter, in the third case the transmission is carried out by modulating the artificial light, so that the lack of reception of a signal within a pre-established time is indicated only by the flashing of the display. It is apparent that such a solution still implies a check by the operator, and therefore can not be accepted for a system intended to reduce to the utmost the need for labour.
Examples of remote displaying devices with two-way transmission are disclosed in GB patent 2.231.994 and in the international (PCT) patent applications WO 90/14630, WO 92/08213 and WO 92/16901. The kind of transmission preferably used is the IR one, since it is immune from electromagnetic interferences, but also the RF transmission is possible as disclosed in U.S. Pat. No. 4,937,586. However, in this case the transmission from the nodes to the labels takes place by means of horizontal loop antennae which must effectively surround the whole area of the supermarket, with the cabling and positioning problems deriving therefrom. Even if other types of antennae are used for said transmission, these problems are not completely solved, particularly because the labels may be located in "shadow regions" in which the reception of the RF signal from the node may be very difficult. On the other hand, the transmission power can not be excessively increased because interferences out of the building could be generated. Moreover, the frequencies available for these transmissions are generally comprised in the band from 10 to 500 KHz, as indicated in the above-cited U.S. Pat. No. 4,937,586. In particular, from the node to the labels it is advantageous to employ a carrier of about 450 KHz to reduce the disturbances by the environmental "noise", while in the opposite direction lower frequencies (about 150 KHz) are employed to reduce the consumption of the RF transmitter of the label. In this way, since the reply message is very short and simple it does not suffer from disturbances and does not require a lot of energy, also because the node is in an exposed and easily reachable position. However, there is the drawback that the RF receiver of the label has a relatively high consumption, since it must operate at frequencies higher than 400 KHz.
Also IR transceiving apparatuses, however, are not exempt from problems, especially due to the lower energetic efficiency of said type of transmission with respect to the RF one. In particular, the problem of high consumption is a serious drawback for the reply transmission from the labels to the nodes, whereas the latter do not have consumption problems because they are fed through the electric lighting. It should also be considered that the reflection of IR signals by the objects is lower than that of RF signals, so that the nodes must be provided with a plurality of IR transmitters oriented in all directions to be sure to reach all labels (e.g. see WO 92/16901). Clearly, the same propagation problem is present, though to a lesser extent, in the transmission from label to node, thus involving the need, in order to reduce said problem, to orient the label properly and/or to provide it with more differently oriented IR transmitters, with a considerable energy increase.
Since the nodes are located on the ceiling or a little lower while the labels may be positioned even at a few centimeters above the floor, it results that the distance to be covered by the reply transmission may reach even 10 meters or more, depending on the number of labels controlled by each node. As an indication, it may be considered that while a LED consumes 1-5-2 A for an IR transmission over such a distance, an RF transmitter requires not more than 10 mA and with a few tens of mA it can reach about a hundred meters.
These energy requirements decisively affect the performances of the labels, which are usually operated by a battery (as in WO 90/14630) possibly recharged by solar cells (as in GB 2.231.994 and WO 92/16901). It is clear that the solution of the battery alone not only poses greater limits on the consumption, but also periodically implies an expensive and long operation for replacing the exhausted batteries on the great number of labels which make part of the system, and which can be several thousands for supermarkets of a certain size (e.g. 20.000). Even rechargeable batteries, though they have a longer life, progressively deteriorate for the repeated charging and discharging cycles, finally requiring a replacement. Moreover, the battery is not able to emit a peak current like the one required by the IR LED, as explained above, so that it is also necessary to have a capacitor which accumulates the energy to be discharged at the moment of the transmission, which moreover can not be repeated until the capacitor has not been sufficiently recharged. Therefore it is apparent that the presence of the battery and capacitor make each label quite expensive and cumbersome.
The only solution proposed in the above-cited documents to attenuate the consumption problems of the label transmitting in IR is the increase in the number of nodes of the communication network so as to reduce the transmission distances. In the case of the system disclosed in the above-mentioned application WO 90/14630, there is considered that each node controls 50 to 75 labels, so that the network is made up of nodes spaced 60-120 centimeters one from the other. This leads to a very high number of nodes, for example at least 300 nodes to control about 20.000 labels, with the huge costs involved in the manufacturing and installation of such a network. The great number of nodes also makes the control and maintenance of the system more expensive, and affects its overall reliability.
A further drawback of the prior art two-way systems stems from the use of the same kind of transmission, either in IR or RF, for the communications from the nodes to the labels and for those in the opposite direction. Since the kind of transmission is the same, a signal emitted from a node and reflected by the underlying objects may be picked up by the receiver of the same node which could try to interpret it as a label reply, or it could interfere with the reply signals transmitted by the labels. This problem is enhanced by the fact that the transceiving units used in the nodes and in the labels are preferably the same, for saving purposes. For example, the device disclosed in the above-cited application WO 92/16901 includes, both in the nodes and in the labels, IR transmission LEDs operating at wavelenghts from 840 to 980 nm and receiving photodiodes which have the greatest sensitivity at the same wavelengths. It is apparent that this can cause interferences between the transmissions in the two directions and the environmental reflections of said transmissions.
A possible solution to this drawback is proposed in the above-cited patent GB 2.231.994. It consists in switching off the receiver for a pre-established period of time after the transmission of the signal, so that possible reflected signal are not misinterpreted by the receiver. However, this arrangement implies the evaluation of the time required for a signal to be reflected towards the source, said time being variable depending on the objects present along the direction of emission of the signal. Therefore it is clear that it is not possible to calibrate the system as a whole, rather it is necessary to calibrate each single transceiving apparatus according to its particular environmental operating conditions. Obviously, this is a very unpractical procedure in the case of systems other than small-sized ones, which moreover has to be repeated when the environment in which the transmissions travel is modified, for example by moving some shelves, etc. Furthermore, it may happen that the reply signal of a label near the node is emitted within a time shorter than that required for the reflection of a transmission signal by an object far from the node, so that the node receiver could be unable to receive the label reply because it is still switched off.